I contacted HP email support, and received this reply -
Thank you for contacting HP Total Care.
I understand from your email that 2 x 250 external hard drives are not recognised in your Pavilion a1555a Desktop PC.
I am glad to help you in this regard.
Rob, this issue might be due to corrupt USB drivers.
To resolve this issue, I recommend you download and update BIOS. (Link for this PC).
1. At the File Download screen, click the Save button.
2. At the Save As screen, save the file to your Desktop.
3. If a message appears stating that the file already exists, click YES to overwrite file.
4. Double-click the icon for the update downloaded to your desktop, and then click NEXT.
5. Follow on-screen instructions.
6. If prompted to restart the system upon completion of the update, click YES and allow system to restart.
After installing BIOS update, I recommend you reinstall USB drivers by performing the steps mentioned below:
1. Click Start and select Control Panel.
2. Double-click the system icon. If the System icon is not found
directly under Control Panel, click the Performance and Maintenance
icon and then click the System icon.
3. The System properties Window opens. You can open the System
Properties window directly by pressing the Windows key and
Pause/Break key simultaneously.
4. Click the Hardware tab and then click the Device Manager button.
5. Click the plus (+) sign next to the Universal Serial Bus controllers.
6. Right-click the USB device under this Category and select
Uninstall. In the Confirm Device Removal dialog box, click OK.
7. Remove all the additional USB devices in the same manner.
8. Restart the computer.
The computer will auto detect the USB device and install the required drivers to run the device
NOTE: If the system prompts for the Windows XP CD, specify the
location as C:\Windows\i386 or C:\Windows\i386\drivercache.
Upon the completion of these steps, the USB device should work normally.
If issue persists, then please reply to this message and we will be happy to assist you further.
For information on keeping your HP and Compaq products up and running, please visit our Web site at: http://www.hp.com/go/totalcare
Have a Nice Day. --
I received an error msg while following these steps and had to do a couple of hard reboots when Windows refused to shut down, but persisted and all seems good now with all drives recognised.